In recent years there has been growing interest in the use of organic polymers for electronic applications. For example, organic polymers have shown promise as the active layer in organic based thin film transistors and organic field effect transistors (TFT, OFET) (see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359). Such devices have potential applications in smart cards, security tags and switching elements in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semi-conducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
Regioregular head-to-tail poly(3-hexyl-thiophene) (P3HT) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2V−1 s−1, but with a rather low current on/off ratio between 10 and 103 (see Z. Bao et al., Appl. Pys. Lett., 1996, 69, 4108). This low on/off current is due in part to the low ionisation potential of the polymer, which can lead to oxygen doping of the polymer under ambient conditions, and a subsequent high off current (see H. Sirringhaus et al., Adv. Solid State Phys., 1999, 39, 101).
A high regioregularity leads to improved packing and optimised microstructure, leading to improved charge carrier mobility (see H. Sirringhaus et al., Science, 1998, 280, 1741-1744; H. Sirringhaus et al., Nature, 1999, 401, 685-688; and H. Sirringhaus, et al., Synthetic Metals, 2000,111-112, 129-132). In general, poly(3-alkyl-thiophenes) (P3AT) show improved solubility and are able to be solution processed to fabricate large area films. However, P3AT have relatively low ionisation potentials and are susceptible to doping in air.
Another particular area of importance is organic photovoltaics (OPV). Organic polymers have found use in OPVs as they allow devices to be manufactured by solution-processing techniques such as spin casting, dip coating or ink jet printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. Currently, polymer based devices are achieving efficiencies up to 4-5% (see for example K. M. Coakley and M. D. McGehee, Chem. Mater. 2004, 16, 4533-4542). This is appreciably lower than the efficiencies attainable by inorganic devices, which are typically up to 25%.
The class of polymers currently achieving the highest efficiencies in OPV devices are P3AT. The most commonly used example is P3HT, due to its broad availability and good absorption characteristics. However, P3HT absorbs strongly over the range from 480 to 650 nm, with a peak maximum absorption at 560 nm. This means a significant portion of the light emitted by the sun is not being absorbed, which is a disadvantage for use in OPV devices. In order to improve the efficiency of OPV devices, polymers are required that absorb more light from the longer wavelength region (650 to 800 nm) of the solar spectra. For this purpose, polymers are desired which have a low band gap, preferably less than 1.9 eV, whereas for example P3HT has a band gap of ˜2.0 eV.
One strategy to further improve photovoltaic efficiency is the development of polythiophene analogues that can capture more of the photon flux from the solar spectrum. This is achieved by reducing the bandgap of the semiconducting polymer. A common strategy to reduce the bandgap of the polymer is the donor-acceptor approach, whereupon electron-rich and electron-poor units are co-polymerised in a polymer backbone. The polymer tends to take the HOMO of the electron rich unit and the LUMO of the electron poor unit, resulting in a low overall bandgap (see Roncali, J. Chem. Rev. 1997, 97, '73) However, an undesirable consequence with this approach is a reduction in the open circuit voltage of the device (VOC), and therefore a reduction in cell efficiency (see Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006, 88, 093511) The VOC in bulk heterojunction devices is derived primarily by the energy difference between the highest occupied molecular orbital (HOMO) of the donor, and the lowest unoccupied molecular orbital (LUMO) of the acceptor (see Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789) Therefore the above-mentioned strategy to reduce polymer bandgap has the disadvantage of raising the polymer HOMO level (the HOMO level is moved closer to vacuum level).
In McCulloch et al., Nature Materials 2006, 5(4), 328-333 it has been reported that polymers incorporating thieno[3,2-b]thiophene exhibit very high charge carrier mobilities (up to 0.6 cm2/Vs) in organic field effect transistors. However, whilst these polymers are useful for OFET devices, the maximum absorption wavelength (550 nm) of the polymer is not optimal for use in OPV devices. As mentioned above, for these devices it is desired to capture the maximum amount of energy from the sun, which has a maximum photon flux around 700 nm.
It is an aim of the present invention to provide new and improved materials for use as semiconductors or charge transport materials, especially for use in OPV and OFET devices, which do not have the disadvantages as mentioned above, and are easy to synthesize, have high charge mobility, good processability and oxidative stability. Another aim of the invention is to provide new semiconductor and charge transport components, and new and improved electrooptical, electronic and luminescent devices comprising these components. Other aims of the invention are immediately evident to those skilled in the art from the following description.
The inventors of the present invention have found that these aims can be achieved by providing polymers comprising fused selenophene rings as described hereinafter. Surprisingly it was found that, especially polymers containing fused selenopheno[3,2-b]thiophene and selenopheno[3,2-b]selenophene, have smaller bandgaps in comparison to the analogous all-sulfur systems. In addition, the inventors of the present invention have found that the inclusion of selenopheno[3,2-b]thiophene into polymer backbone surprisingly results in an improvement of the solubility of the polymer without affecting the ability of the polymer to self-organise and pack into highly ordered films. (see comment also on page 18)
JP 2006-089413 A discloses low molar mass compounds comprising a selenopheno[3,2-b]selenophene group, but does not disclose polymers according to the present invention. In addition JP2006089413A does not disclose selenopheno[3,2-b]selenophene or selenopheno[3,2-b]thiophenes that are substituted in the 3,6-positions with solubilising sidechains, or methods to prepare such compounds. Unsubstituted selenopheno[3,2-b]selenophene in combination with aromatic units containing solubilising sidechains are also not disclosed. The use of such sidechains is important, not only in providing solubility to the polymer, but also in driving self-organisation of the polymer backbone by interdigitation of the sidechains on adjacent polymer backbones. This self-organisation allows closer packing of the polymer backbones and is critical in enabling the polymer to pack closely in transistor devices, since charge is carried by a hopping mechanism from one molecule to an adjacent molecule. In addition, organic photovoltaic cells utilise a blend of a p-type and n-type component that must phase separation on the nano-scale in order to promote efficient excitation diffusion to the interface, splitting of the excitation into holes and electrons and subsequent collection of the charges at the respective electrodes (see Padinger, J.; Rittberger, R. S.; Sariciftci, N. S. Adv. Funct. Mater. 2003, 13, 85) Without phase separation this process is inefficient resulting in low performing cells. The inventors of the present invention have found that the self-organisational abilities of the presently reported polymers promotes such phase separation in an annealed blended film with typical n-type component such as PCBM (6,6}-phenyl C61-butyric acid methyl ester).